Underground combustion operation



NW. 13, 1956 s. .J. PRRSON EJ7013 UNDERGROUND COMBUSTION OPERATION Filed Sept. 2, 1952 .INVENTOR.

SYLVAIN J. PIRSOM ATTU/M/EV UNDERGROUND COMBUSTION OPERATION Sylvain J. Pirson, Tulsa, Okla., assignor to Stanoliud Oil and Gas Company, Tulsa, Okla., a corporation of Delaware Application September 2, 1952, Serial No. 307,548

4 Claims. (Cl. 166--4) This invention relates to the recovery of oil from underground reservoirs, particularly by a method involving combustion of a portion of the oil in place. More particularly, it is directed to the more efficient carying out of the combustion process, by measuring the position of the combustion front periodically and varying the oxygen input rate to maintain the desired rate of progress of the combustion front.

The recovery of oil by combustion of a portion of the oil in place to generate heat and products of combustion, which together drive out substantially all of the oil in a given reservoir, except that consumed in the process, has been proposed and tried on a minor scale in several instances. One dificulty, however, encountered in the field operation of a combustion process is in determining the necessary amount of oxygen to maintain the combustion front in motion with at least a minimum velocity. Lacking exact knowledge of the front position at any given time, expensive additional equipment is required to provide a margin of safety that allows for errors in estimating the front position on the basis of general engineering data. It has been recognized that, if the exact position of the front can be ascertained at any desired time, the oxygen input rate can be maintained much closer to the absolute minimum value required for successful operation, thereby controlling the character of the combustion to some extent as well as reducing the equipment investment required by a substantial fraction, particularly when the dimensions of the front become very large.

In view of the foregoing it is an object of my invention to provide, in a process of underground combustion, a highly elficient method of following the combustion front as it progreses through a reservoir, so that the rate of oxidant injection can be regulated to the necessary value with some precision. Another object is to provide a method of measurement of the burned-out area in an underground combustion project, which method may be carried out entirely at the earths surface at any time desired. A further object is to provide a method of utilizing, for the measurement of area of an underground combustion zone, certain magnetic properties of the rock which are affected by the passage of the combustion front. Other objects, uses and advantages will become apparent as the description of the invention proceeds.

The foregoing and other objects of my invention are accomplished by periodically making, before and during the progress of an underground combustion operation, detailed magnetic surveys at the earths surface above the reservoir. I have found that many oil-bearing sedimentary formations contain sufficient materials having magnetic properties capable of being altered by the passage of a combustion front to produce substantial changes in the earths magentic field measured at the surface. These changes are directly interpretable in terms of the area through which a combustion front has passed, so that the actual surface area in the front itself can be estimated with fairly good accuracy, and the oxygen input rate can nited States PatentO then be regulated accordingly. A further feature of the invention is the fact that as the burned-out area becomes larger, the indications of the boundary become more and more precise, so that when more accurate knowledge of the exact oxygen-input requirements is most necessary to avoid overloading the available equipment at the highest oxygen injection rates, the required measurements are available or can be obtained.

This will be better understood by reference to the accompanying drawing forming a part of this application, which drawing shows diagrammatically and partly in cross-section a portion of the earth containing an oilbearing stratum penetrated by an oxygen-input well surrounded by a combustion area. Superimposed above this cross-section with the same horizontal scale is a plot of surface magnetic anomalies produced by the combustion in the underground stratum at correspondingly marked different combustion-front positions shown in the stratum.

Referring now to this drawing in detail, a well 10 extends from the earths surface 11, penetrating subsurface layers 12 and terminating in an oil-bearing stratum 13. As the particular arrangement of oxygen-injecting and oilrecovery equipment, or of input and output wells, is not important in the present invention, only a single input well 10 is shown. In accordance with the invention, however, prior to the beginning of injection of oxygen into well 10, a magentic survey is made at surface 11 in all directions around well 10. This is preferably done after all of the equipment to be used in the operation is in place in the well or wells and at the surface. The survey may comprise measuring either the total magnetic field or any of its components, horizontal or vertical. Due to the generally small difference between the magnitude and direction of the total field and its vertical component in the United States, it is generally preferred to measure the latter; and in the example shown, it will be assumed that this has been done.

This survey is carried out either by continuous measurements along a plurality of closely-spaced, parallel lines covering and extending somewhat beyond the subsurface reservoir area, or along roughly radial lines extending out from each oxygen-input well. If the equipment used in carrying out this survey is not adapted for continuous recording, points along the corresponding lines are measured at fairly closely-spaced intervals. Preferably all measurements are made with reference to a fixed base station completely remote from the reservoir area. The measurements are carefully made and surveyed, and preferably markers are set identifying the lines or points of measurement so that they can be subsequently reoccupied.

Following the first complete magnetic survey of the area, oxygen injection is then begun through well 10, ignition of the oil in place at the well bottom is accomplished, and the combustion front 14 is propagated through the formation 13 radially outward from well 10. The rate of propagation of the front 14 from well 10 is estimated as closely as possible from a knowledge of the amount of oxygen injected and of the combustion characteristics of the oil and reservoir rock. At such times, however, as it is desired to ascertain more exactly the area which has been covered by the underground combustion front, a magentic survey of the surface 11 is again performed, occupying preferably the same measurement points as in the original survey, and utiilzing preferably the same off-structure base station. For each point or line of measurement, the difference is then ascertained between the original measured values of the magnetic field at the surface 11 and the remeasured value at each point or along each line. Appropriate correction factors are employed for the readings before taking the differences, both to compensate for variations of basestation magentic field during the period of each survey and between the two surveys.

The resulting differences, reduced thus to a constant standard base-station reading, are plotted on a map of the area, or profiles are plotted along radial lines if that is the plan of measurement. Referring again to the drawing, the curve A plotted to the horizontal scale of the earth crosssection and with reference to the Zero ordinate line marked is the anomaly profile which is obtained along a radial line extending on either side of wall when the combustion front reaches the positions A1 and A2 in the oil-bearing stratum 13. Knowing the depth and thickness of stratum 13 and the characteristic change in magnetic properties to be expected, which is readily obtained by subjecting a core from the stratum to combustion in a laboratory apparatus, it is then a straightforward matter of interpretation of the animaly curve A to arrive at the area traversed by front 14 out to positions A1 and A2 in the stratum 13. As long as the distance from A1 to A2 on opposite sides of the well 10 is small compared to the thickness of the subsurface beds 12, i. e., the depth of the stratum 13, the anomaly curve A has the general form shown, and the height or maximum value of the anomaly approximately above the position of well 10 varies directly with the average distance the combustion front 14 has traveled. Where the inclination of the earths field departs considerably from the vertical, the position of the maximum anomaly is corespondingly shifted away from the central position shown, but this effect can ordinarily be considered negligible.

The curves B and C represents qualitatively the anomalies to be expected when the combustion front 14 extends between the limits B1 and B2 and C1 and C9. respectively. As the front 14- progresses radially outward to distances such as D]. and D2, which are substantially larger than the depth of the stratum 13, the maximum value of the anomaly no longer increases rapidly in magnitude over well Ml, but the curve flattens out in the center as shown in curve D where the edge effects at the fronts D1 and D2 becomes relatively more significant. Also, it is to be noted that as the front 14 moves outwardly, the horizontal distances to the intercepts A, B, C and D, at which the anomaly curves pass through zero in changing from positive to negative values, approach the actual distance of the front at D1 and D2 from the well 10.

A simple approximation, providing the inclination of the field from the vertical is not too great, therefore comprises utilizing the zero intercept-s of the anomaly curvesrespectively A, B, C and Das the actual positions of the underground front at the respective times. This results in the front at A1 and A2 being estimated larger than it actually is, so that oxygen is injected at a higher rate per unit of front area than the minimum. Nevertheless, as the area in the burning face is small for small radial distances from well lit, this does not overtax the oxygen supply system. As the front moves outwardly, and more and more oxygen is needed to be injected through well it the discrepancy between the assumed distance D and the actual position D1 becomes smaller and smaller, so that the assumed oxygen rate becomes more closely the actual necessary rate to supply the desired minimum per square foot of front area. As an example, tests have shown that when a combustion drive operation is to be carried out in a formation having about the characteristics of torpedo sand (porosity 20 to 23 percent and permeability 500 to 1000 millidarcies), the rate of oxygen supply should be at least 10 s. c. f. (standard cubic feet) per hour per square foot of front area. If the rate falls much below this for too long a period of time, the combustion is apt to die out and cannot easily be reestablished.

The reason for the appearance of a magnetic anomaly which can be used to outline the area through which underground combustion has passed seems to be that the temperature of the combustion front and the chemical and physical reactions occurring therein change the magnetic state of the small amounts of iron present in many subsurface minerals to a more highly magnetic condition. That this is true is borne out in part by the following results of measurements made on samples before and after being subjected to a combustion drive to force out hydrocarbons. For a Nellie Bly sandstone sample the remanent magnetism was changed by combustion from 14x10 to 230 l() c. g. s. units, while the susceptibility was changed from zero to 7 8X10" c. g. s. units. Similarly, the remanent magnetism of a Torpedo sandstone sample was altered by the combustion from 1.3 l0 to 280 lO units, and its susceptibility changed from 9 1() to 43 l()- c. g. s. units. In both cases the peak temperature attained was about 1000 P.

On the basis of these data, assuming an average change in total magnetic polarization due to the changes in both the remanent magnetism and the magnetic susceptibility of 2500x10 c. g. s. units, the following approximate vertical-component anomaly values can be readily computed, assuming also the entire polarization to be vertical, which is not far from true:

This table emphasizese also the fact that, for a given change in polarization, the magnitude of the surface anomaly depends substantially only on the difference in solid angles subtended by the top and bottom of the burnedover area. That is to say, a lO-foot bed of -foot radius at IOOO-foot depth gives the same maximum magnetic anomaly as a 20-foot bed of ZOO-foot radius at 2000 feet, as the solid angles involved are the same.

If a more exact determination of the shape and size of the burned-through area is desired, a more accurate interpretation of the anomaly profiles obtained by plotting the differential measurements as set out above may be made on the basis of the method suggested by L. J. Peters and described in Geophysics, vol. XIV (1949), pp. 290 320.

While I have thus described my invention in terms of the foregoing specific embodiments and modifications, it is to be understood that the scope of the invention is not limited tothe details set forth but is to be ascertained by reference to the appended claims.

I claim:

1. In the recovery of oil from an underground reservoir by a process involving combustion of a portion of the oil in place, wherein the reservoir stratum, at a known depth and of known thickness, contains materials having magnetic properties susceptible to being altered by the heat of the combustion process, and wherein the rate of oxygen supply to a combustion front in said stratum must he maintained above a minimum value per unit of front area to secure propagation of said front therethrough in a stable manner, the improvement comprising, in combination, the steps of making at the earths surface an initial set of measurements of the earths magnetic field intensity along a plurality of profile lines extending across at least the surface area overlying the portions of said straturn to be subjected to combustion, initiating a combustion front therein and maintaining combustion in said stratum by supplying an oxygen-containing gas to said front through at least one oxygen-input well penetrating said stratum, rcmeasuring at intervals the earths magnetic field intensity, and determining and plotting the differences between the initial and the remeasured values of said magnetic field intensity so as to show in several directions around said input well the positions of the boundary of the burned-out zone, noting said boundary positions, and adjusting, on the basis of said reineasured values, the rate of injection of said oxygen-containing gas into said input well so as to maintain the rate of oxygen supply to said front at least equal to said minimum value per unit of front area.

2. A process according to claim 1 in which the magnetic field intensity measurements comprise measuring the vertical component of the earths magnetic field.

3. A process according to claim 1 in which said profile lines extend substantially radially outwardly from said oxygen input well.

4. A process according to claim 1 in which said difference-plotting step is so perfomned as to show in several directions around said oxygen-input well the points at which said difierences decrease to Zero, and said calculating step is carried out assuming that said boundary lies exactly under said points.

References Cited in the file of this patent UNITED STATES PATENTS 

